1. Field of the Invention
The present invention relates to a cylinder device and, specifically, it relates to a cylinder device structured such that a piston is movably inserted into a cylinder through a cup seal, for example, a hydraulic master cylinder of a plunger type for a vehicle.
2. Description of Related Art
Conventionally, there is known a cylinder device structured such that, a annular seal groove is formed on an inner circumferential surface of a cylinder within which a piston is inserted and a cup seal is fitted into the seal groove. Herein, the cup includes a base portion to be fitted with the cylinder hole opening side surface of the seal groove, an inner circumferential lip extending from the inner circumferential side of the base portion of the cup seal to the bottom portion side of the cylinder hole of the cylinder and an outer circumferential lip extending from the outer circumferential side of the base portion of the cup seal to the bottom portion side of the cylinder hole (see, for example, Japanese Patent Unexamined Publication No. JP-A-2004-231093).
Also, as a hydraulic master cylinder of plunger type, there is known a structure in which, a projection projecting from the front end of the inner circumferential lip is formed on the inner circumferential lip of a cup seal. In this structure, when a piston is returned quickly, the projection is pressed against a side wall of the cylinder hole bottom portion of a seal groove and is thus deformed elastically to flex an outer circumferential lip, thereby forming a flow passage for operating fluid between the seal groove and cup seal (see, for example, Japanese Patent Unexamined Publication No. JP-A-2005-186925).
In the conventional cylinder device disclosed in the above-mentioned the JP-A-2004-231093, however, since the cup seal is fitted into the seal groove with a clearance in the axial direction of the cylinder, when the piston is operated, the cup seal can be moved in the cylinder axial direction, thereby raising a fear that the ineffective stroke of the piston is increased or the ineffective stroke of the piston is varied.
Further, in the conventional hydraulic master cylinder of the plunger type disclosed in the JP-A-2005-186925, although the projection is provided on the cup seal to prevent the cup seal from moving in the cylinder axial direction, the cup seal is caused to collapse when the piston is returned quickly. Therefore, when the inner circumferential lip is pressed outwardly in a radial direction due to the flow of the operating liquid and the piston is thus returned to its initial position while the cup seal is left flexed outwardly in the radial direction, there is a fear that a sealing force between the inner circumferential lip and piston is reduced.